Contact image sensors that are used in image reading devices such as copiers or multi-function printers are advancing in terms of increased precision of reading depth and higher resolution. Therefore, in an optical system for a contact image sensor, construction is desired in which the relative positional relationship of a document, a lens and sensor is regulated with high precision. Unexamined Japanese Patent Application Kokai Publication No. H07-131585 (see Patent Literature 1) discloses technology in which this relative positional relationship is regulated with high precision by installing a rod lens, a document glass and a sensor-mounted board as optical elements on a high-precision member that is formed by aluminum extrusion molding or pressing of steel plate.
Moreover, in the optical system of a contact image sensor, light that is reflected from the document, passes through a lens and reaches the sensor, is converted by the sensor to an electrical signal so that information in the document is converted to the electrical signal. However, when there is divergence of the optical axis, irregular reflection of light occurs by the light that passes through the lens hitting a location other than the sensor, and that light arrives at the sensor as stray light, causing the image reading performance to become poor. Unexamined Japanese Patent Application Kokai Publication No. H08-307597 (see Patent Literature 2) discloses an imaging device in which a rod lens array that focuses the light rays that are reflected from the surface of the document is held between a frame and a holder. The holder keeps the light rays that are irradiated by an LED chip from directly entering into a light-receiving sensor. As a result, this imaging device prevents divergence of the optical axis and suppresses stray light.
As the light output from light sources of contact image sensors becomes greater, there is a need for efficient heat-dissipating construction. Unexamined Japanese Patent Application Kokai Publication No. 2008-227815 (see Patent Literature 3) discloses, as illustrated in FIG. 4, heat-dissipating construction in which an LED chip is mounted on a metal plate, and bent heat-dissipating plates are attached to a flat section on the reflecting-surface side of the LED chip.